lib, ovs-vsctl: Add zero-initializations.

This commit adds zero-initializations by changing `SFL_ALLOC` from
`malloc` to `xzalloc`, adding a `memset` call to `sflAlloc`,
initializing a `pollfd` struct variable with zeroes, and changing some
calls to `xmalloc` to `xzalloc`. This is to prevent potential data leaks
or undefined behavior from potentially uninitialized variables.

Some variables would always be initialized by either the code flow or
the compiler. Thus, some of the associated Coverity reports might be
false positives. That said, it is still considered best practice to
zero-initialize variables upfront just in case to ensure the overall
resilience and security of OVS, as long as they do not impact
performance-critical code. As a bonus, it would also make static
analyzer tools, such as Coverity, happy.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/sflow_api.h

@@ -337,7 +337,7 @@ void sfl_agent_sysError(SFLAgent *agent, char *modName, char *msg);
 
 u_int32_t sfl_receiver_samplePacketsSent(SFLReceiver *receiver);
 
-#define SFL_ALLOC malloc
+#define SFL_ALLOC xzalloc
 #define SFL_FREE free
 
 #endif /* SFLOW_API_H */